schurr r



Dec. 15, 1959 v, D, SCHURR Re. 24,756

HIGHLY STABLE ELECTRONIC AMPLIFIER Original Filed Dec. 21, 1951 3Sheets-Sheet 1 INVENTOR Vernon Dale Jcfi. arr.

v. D. scHuRR HIGHLY STABLE ELECTRONIC AMPLIFIER 3 Sheets-Sheet 2 Dec.15, 1959 Original Filed Dec. 21, 1951 35 INVENTQR Vernon Date Jail??-Dec. 15, 1959 v. D. SCHURR ,7

HIGHLY STABLE ELECTRONIC AMPLIFIER Original Filed Dec. 21, 1951 3Sheets-Sheet 3 nun United States Patent Ofiice Re. 24,756 Reissued Dec.15, 1959 HIGHLY STABLE ELECTRONIC AMPLIFIER Vernon Dale Schurr,Linfield, Pa., assignor, by direct and mesue assignments, to TechnitrolEngineering Company, Inc., Philadelphia, Pa., a corporation ofPennsylvania Original No.'2,746,016, dated May 15, 1956, Serial No.262,694, December 21, 1951. Application for reissue May 12, 1958, SerialNo. 735,095

8 Claims. (Cl. 324-123) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

The present invention relates to highly stable electronic amplifiers,especially instrument amplifiers.

A purpose of the invention is to create a direct coupled highly stableelectronic amplifier, especially an instrument amplifier, suited for avoltmeter, ammeter, ohmmeter or electronic computer.

A further purpose is to obtain a large input voltage range with infiniteinput impedance throughout the voltage range with negligible inputcurrent. I

A further purpose is to hold the voltage from plate to cathode, theplate current, and the voltage from grid to cathode of the input vacuumtube means constant with change in the input voltage.

A further purpose is to avoid input voltage dividers.

A further purpose is to obtain automatic adjustment of bucking voltage.

A further purpose, as the positive voltage on the control grid of theinput vacuum tube increases, is to make the voltages of the cathodes ofthe input and amplifier vacuum tubes increase, securing a regenerativeaction from a separate vacuum tube to the cathode of the amplifiervacuum tube which causes the amplifier vacuum tube to have infinite gainand applying the same regenerative action to the cathode of the inputvacuum tube, thence to the control grid of the amplifier vacuum tubewhich likewise again influences the amplifier vacuum tube.

A further purpose is to produce a combined effect of positive andnegative feed back which causes the cathode.

and anode of the input vacuum tube to rise in voltage an amount equal tothe increase in input voltage so that the potentials and currents in theinput vacuum tube remain the same as when the input voltage is zero.

A further purpose is to employ an input vacuum tube, an amplifier vacuumtube and means to produce positive and negative feedback from theamplifier vacuum tube to the input and amplifier vacuum tubes tomaintain the potentials and currents in the input vacuum tube the samewhen the input voltage changes, to couple from the anode of theinputvacuum tube to the control grid of the amplifier vacuum tube, toconnect from the anode of the amplifier vacuum tube to a feedback vacuumtube and suitably to the control grid of the feedback vacuum tube, toconnect from the output of the feed back vacuum tube to the cathodes ofthe input and amplifier vacuum tubes and to provide a bucking voltage tothe anode of the feedback vacuum tube.

Further purposes appear in the specification and in the claims.

In the drawings I have chosen to illustrate a few only of the numerousembodiments in which my invention may appear, selecting the forms shownfrom the standpoints of convenience in illustration, satisfactoryoperation and clear demonstration of the principles involved.

The drawings arediagrams of circuits useful in explaining the invention.

The present invention is concerned with a direct current or alternatingcurrent amplifier which is suitable as a directly coupled highly stableamplifier useful in any amplifier application, but particularly intendedfor instrumentation as in the operation of voltmeters, ammeters,ohmmeters and for electronic computers and control devices. The deviceof the invention has the distinctive feature that the amplifier has avery large input voltage range and infinite input impedance throughoutthe input voltage range. This means effectively that the amplifier willoperate over a wide input voltage range with very high input impedance.In conventional amplifiers input impedance changes throughout the inputvoltage range.

By the invention it is possible to measure simple differences in voltageat high voltage levels with the same accuracy that you would measuresmall differences in voltage at low voltage levels. Thus, for example,the device of the invention will measure the difference between voltsand 100.01 volts with the same accuracy that it will measure thedifference between 0.01 volt and 0.02 volt.

'The invention is also applicable in sensitive control circuits, sincethe device has a minimum effect on the measuring circuit itself,eliminating expensive potentiometers or resistance networks.

In the prior art the so called slideback voltmeter has been used, whichis adjusted to provide the same conditions in the input tube with afinite input voltage as when the input voltage is zero. This device,however, requires tedious adjustment, and does not automatically adjustto maintain constant conditions in the input vacuum tube.

The cathode follower of the prior art provides simplicity, low outputimpedance, fair linearity and high input impedance, but it cannotaccommodate a large input range unless a high voltage from plate tocathode is provided where the input voltage is zero. This high voltagefrom plate to cathode together with the relatively high plate currentwill not allow the input vacuum tube to operate throughout its rangewith minimum grid current. As the input voltage is increased the voltagefrom grid to cathode decreases in spite of the name cathode follower.

Feedback amplifiers which incorporate negative feedback are stable andhave an output impedance even lower than the cathode follower. How-ever,these devices like the cathode follower require that the voltage fromplate to cathode must be several times the maximum input voltage,leading to the difiiculty mentioned with the cathode follower of theprior art. The 100 percent negative feedback amplifiers with highamplification have a constant plate current of the input tube, but thevoltage from plate to cathode and from grid to cathode changes with theinput voltage and therefore does not assure minimum grid current.

In accordance with the present invention there is antomatic maintenanceof constant conditions of voltage and current in the input vacuum tubeso that the operation of the input tube does not change as the inputvoltage changes.

In accordance with the invention, a bucking voltage is automaticallyadjusted. As the positive voltage on the control grid of the input tubeincreases, the voltages of the cathodes of the input and amplifiervacuum tubes increase. Regenerative action takes place from a feedbacktube effectively to the control grid of the amplifier tube which causesthe amplifier tube to have infinite gain. This effect is applied to thecathodes of the input tube and the amplifier tube, and the input tubeamplifies the effect and returns it to. the control grid of theamplifier vacuum tube in the proper phase to maintain stability.

Thus there is combined elfect of positive and negative feedback whichcauses the cathode and anode of the input vacuum tube to increase involtage by an amount equal to the increase in input voltage. Thus thepoten tials and currents in the input tube remain the same as when theinput voltage was zero. The device. therefore behaves as an infiniteinput impedance device throughout the working range of input voltage.The voltage from plate to cathode and from grid to cathode and the platecurrent of the input tube remain constant with change in input voltage.

h mp fi he n ion may e u ed e ie vely on either alternating or directcurrent.

In accordance with the invention, I have succeeded in pro ng an mpl fierwhi has n inpu an of .0 o +150 lts ith y high np imp danc nd e peei l yo gr d, curren o er e f l ran Th s s ac complished using ordinaryreceiving vacuum tubes and non-precision resistors. For a higher rangeof input, the supply voltages can be increased, and for; a higher inputimpedance an electrometer tube may be used for the input tube.

Since a large plate to cathode voltage is ordinarily necessary for alarge input range and a low plate to cathode voltage is required for aminimum current from grid to cathode, it would be very desirable to havea low plate to cathode voltage and hold it constant as the input voltagechanges. If a positive feedback amplifier were con,- nected to the inputtube with the amplifier connected to the output terminal it would act asa positive feedback amplifier with more than the critical amount oipositive feedback, and as such would have two stable states limited bythe supply voltage and ground. This condition would be prevented by theinput vacuum tube since any change in the output voltage would beamplified by the input vacuum tube and applied to the amplifier in the,same direction as the output voltage, thus locking the input vacuum tubeand the amplifier into a stable state and giving the amplifier aninfinite gain. Under this condition the input voltage can change withoutthe plate to cathode voltage of the input vacuum tube changing, sincethe plate to cathode voltage can only be changed by altering the designof the amplifier.

While this would be an improvement over the prior art, the grid tocathode voltage of the input vacuum tube still changes with the inputvoltage although to a lesser degree. In this case the grid to cathodevoltage becomes more negative with the increase of input voltage due tothe positive feedback of the amplifier. If new the plate current is heldconstant in addition to. the voltage from plate to cathode, the voltagefrom grid to. cathode will not change with any change of the inputvoltage. This is accomplished by coupling the anode of the input vacuumtube through a source of voltage to the output terminal, Under thiscondition the input voltage can change without limit, without changingthe voltage from plate to cathode, the plate current or the voltage fromgrid to cathode of the input vacuum tube. Once these three parametersare fixed for minimum grid current the grid current will remain constantover any range of input voltage within the design of the amplifier.

In Figure 1 the input voltage is applied at terminal (positive if theinput is direct current) to the control grid of input vacuum tube 21,having an anode, a cathode and a control grid. The opposite side of theinput (the negative in the case of direct current) is connected togrounded terminal 22.

A signal from the anode of input vacuum tube 21 is carried by lead 23 tothe control grid of amplifier vacuum tube 24 having an anode, a cathodeand a control grid. The anode of input vacuum tube 21 is also connectedthrough plate resistor 25 to the positive side of direct current Bsource 26, the negative side of which is connected 4 to the cathode ofthe input vacuum tube 21 and to output terminal 27 to which voltmeter,ammeter, ohmmeter or other instrument 28 is connected. The opposite sideof the instrument 28 is connected to terminal 30 which is connected tothe positive side of direct current grid bias source 31, the negativeside of which is grounded.

Amplifier vacuum tube 24 has its anode connected by lead 32 to thecontrol grid of cathode follower vacuum tube 33 having an anode, acathode and a control grid. The anode amplifier vacuum tube 24 alsoconnects through a load resistor 34 with the anode of cathode followervacuum tube 33 and also with the positive side of a source of buckingdirect current voltage 35, the negative side of which is connected withthe terminal 30 of the instrument 28 and with the positive side of gridbiasing source 31, the negative side of which is grounded. The cathodeof amplifier tube 24 is connected to the positive side of the directcurrent grid bias source 36, the negative side of which is connected toinstrument terminal 27. The cathode of cathode follower vacuum tube 33is connected to the positive side of direct current grid bias source 37,the negative. side of which is connected to the instrument terminal 27.

It will be evident that in the circuit of Figure 1 there s n. input u mtube, n ampl fier acu m tu e and mean to o p e e u put o th amp ifi rback o he amplifier tube and the input vacuum tube. The anode of theinput vacuum tube is directly connected to the control grid of theamplifier vacuum tube. The anode of the amplifier vacuum tube is alsoconnected to the mea for p ng bac f m. th mpli r o he mplifier and inputvacuum tubes. In this case the connection is made to the control grid ofvacuum tube 33. The means for coupling back is connected to the cathodesof the input and amplifier vacuum tubes, in this case through theinstrument 28, and the source of bucking voltage which is connected atthe positive side to the anode of vacuum tube 33.

In operation a signal is taken from the anode of input vacuum tube 21and applied to the control grid of ampliiier vacuum tube 24 which hasits anode connected to the control grid of cathode follower vacuum tube33. As voltage is applied to input terminal 20 and to the control gridof input vacuum tube 21 there is automatic adjustment of the buckingvoltage supplied by source. 35.

If one assumes that the positive input voltage at ter minal 20increases, the elfect is to. lower the anode voltage on input vacuumtube 21, which is applied to the control grid of amplifier vacuum tube24, and this raises the anode voltage of amplifier vacuum tube 24 whichincreases the current flowing frorn cathode to plate in cathode followervacuum tube 33, which increases the voltage across the voltmeter orother instrument 28, which raises the voltages of the cathodes of inputvacuum tube 21 and amplifier vacuum tube 24. When this action takesplace there is a regenerative action from cathode follower vacuum tube33 applied effectively to the control grid of amplifier vacuum tube 24,which causes the amplifier to have infinite gain. This last result isachieved by the signal applied to the cathode of amplificr vacuum tube24 and of input vacuum tube 21, which is effectively the same as makingthe grid of input vacuum tube 21 more negative and therefore this effectis amplified in input vacuum tube 21 and applied to the control grid ofamplifier vacuum tube 24. This cancels out the tendency to regenerate inthe effect on amplifier vacuum tube 24. The combined actions of thepositive and negative feedback thus produced cause the cathode and anodeof input vacuum tube 21 to raise an amount equal to the increase in theinput voltage. Thus the potentials and currents in input vacuum tube 21are now the same as when the input voltage equaled zero. Thus the risein the input voltage can be measured by the instrument 28 or any othersimilar relation can be meas ured without drawing any appreciablecurrent from the circuit connected to input terminals 20 and 22.

The operation of Figure 1 may be briefly summarized as follows:

The input vacuum tube 21 (input signal amplifying means) receivessignals which it transmits via amplifier vacuum tube 24 (intermediatesignal amplifying means) to cathode follower vacuum tube 33 (outputsignal ampliying means) having an instrument 28 in its cathode circuit(output circuit means). One feedback means couples one terminal of theinstrument 28 to the anode (output element) of input vacuum tube 21. Asecond feedback means couples the same terminal of instrument 28 to thecathode (reference element) of input vacuum tube 21, and a thirdfeedback means couples the same terminal of instrument 28 to the cathodeof amplifier vacuum tube 24. Thus when a signal is fed to the controlgrid (input element) of input vacuum tube 21, a signal of the same phaseis developed across instrument 28. The signal developed acrossinstrument 28 is applied to the anode and cathode of input vacuum tube21 and the cathode of amplifier vacuum tube 24.

The device of Figure 1 has the advantage of being readily portable sinceit is battery operated.

The mechanism of Figure 2 differs from that of Figure 1 in that acentral grid bias battery has been provided which also supplies the Bbattery voltage for the input vacuum tube. In this form the centraldirect current battery source 38 is connected at its positive side tothe cathode of cathode follower vacuum tube 33 and at its negative sideto output terminal 27 connected to the instrument 28 and also to thecathode of input vacuum tube 21. The opposite output terminal 30 in thiscase is connected to an intermediate tap 40 close to the negative sideof the bucking voltage source 35. The negative side of the buckingvoltage source 35 is suitably connected to ground by connection 41.

The common biasing battery 38 has two intermediate taps, the first ofwhich, 42, nearer the positive terminal, is connected through plate loadresistor 25 to the anode of input vacuum tube 21. The second tap 43,closer to the negative terminal of source 38, is connected to thecathode of amplifier tube 24.

The circuit of Figure 3 is a variation which has eliminated the gridbiasing source 38 and employs a gasfilled tube 44 suitably of type VR90interposed between the cathode of cathode follower vacuum tube 33 andoutput terminal 27, with the anode of the gas-filled tube connected tothe cathode of cathode follower 33 and the cathode of the gas-filledtube connected to the cathode of input tube 21, through bias resistor 45to amplifier tube 24, and to voltage source 35 through meter 28. Thegas-filled tube 44 provides the'bias or dropping voltage for the cathodefollower vacuum tube 33 and the means for the back connection of cathodefollower 33. Bias for amplifier vacuum tube 24 is provided by resistor45 interposed between the cathode of the amplifier vacuum tube 24 andoutput terminal 27, to which the cathode of input vacuum tube 21 isdirectly connected. Resistor 45 can also provide the supply voltage forinput vacuum tube 21. The bias for the input vacuum tube 21 is providedby the comparatively low voltage tap 40 across the source of buckingvoltage 35.

The anode of input vacuum tube 21 is connected through plate loadresistor 25 to the cathode of amplifier vacuum tube 24. There is also aphantom connection 46 shown from the anode of input vacuum tube 21through plate load resistor 25 to the cathode of cathode follower tube46 to indicate that the connection 46 is optional for use where moreplate voltage is required for the particular tube used at 21. Phantomconnection 46 will be used without the connection from load resistor 25to the cathode of amplifier vacuum tube 24.

A resistor 47 is shown shunting the output terminals 27 and 30 toprovide conductivity in case the meter 28 is removed. There is no needfor the shunt 47 while the meter is in place.

The amplifier vacuum tube 24 is desirably chosenso that the grid bias issuflicient to raise its cathode high enough to supply the B voltage forthe input vacuum tube as shown.

Figure 4 shows a circuit according to the invention resembling Figure 3,but using an external supply for the direct current bucking voltagehaving the positive side connected at 35 and the negative side at 35This form is also unusual in providing for measurement of input voltagesbelow ground (negative). In this form a second cathode follower vacuumtube 48 having an anode, a cathode and a control grid has beenintroduced. This effectively drives the amplifier vacuum tube 24. Inthis case the anode of input vacuum tube 21 is connected to the cathodeof first cathode follower vacuum tube 33 through plate load resistor 25.Besides the connection 32 from the anode of amplifier vacuum tube 24 tothe control grid of first cathode follower vacuum tube 33, the anode ofamplifier vacuum tube 24- is connected through plate load resistor 34 tothe anode of first cathode follower vacuum tube 33, to the anode ofsecond cathode follower'vacuum tube 48 and to the positive side of thebucking source at 35. The negative side of the bucking source at 35 isconnected through a common bias resistor 50 to the cathodes of theamplifier vacuum tube 24 and the second cathode follower vacuum tube 48.The control grid of the second cathode follower vacuum tube 48 isconnected to the cathode of input vacuum tube 21, to gas tube 44, and tooutput terminal 27. The opposite out put terminal 30 is grounded. Inthis case to provide con ductivity a resistor 51 is placed acrossbetween output terminal 27 and the negative side of the bucking source35 The common bias resistor 50 terminates at the negative side of thebucking source and the amplifier tube 24 is driven by the backconnection from first cathode follower vacuum tube 33 through gas tube44 and second cathode follower 48. Since the driving of a tube from thecathode loads the circuit, by interposing second cathode follower vacuumtube 48 in the circuit, difliculty is prevented through loading of thefeedback circuit by the connection of amplifier vacuum tube 24. Thus thecathodes of the amplifier vacuum tube and the second cathode followervacuum tube can be connected together and through a common droppingresistor 50 to the negative side of the bucking source.

Figure 5 shows a circuit embodying principles of the circuits previouslydiscussed, employing a pentode as the input tube with the usualconnection of the suppressor to the cathode. This may be of tube type617. The amplifier tube will desirably be one-half of a tube of type6SL7. The first and second cathode follower tubes will desirably each beone-half of tube type 6SN7, and the gas tube will be of type VRlSO. Inmany respects this circuit of Figure 5 resembles that of Figure 4 butwith the additional feature that the screen grid of input tube 21 isconnected to the cathode of amplifier vacuum tube 24 and the cathode ofsecond cathode follower vacuum tube 48. A top cap type of input tube isdesirable because the top cap construction reduces the leakage from thecontrol grid to the other elements. An electrometer tube would be evenmore suitable. Thecombinations of the amplifier vacuum tube 24 and thesecond cathode follower tube 48 were chosen to hold the voltage fromplate to cathode in the input vacuum tube at about 22 volts.

Instead of connecting the instrument between output terminals 27 and 30in this case, two identical amplifiers as in Figure 5 are connected inpush-pull relationship to opposite sides of voltmeter 28, the arrow 52indicating connection to a similar circuit at its output terminal 27 onthe opposite side of the voltmeter. This eliminates the need for abucking voltage for the voltmeter, balances out drift caused by thechanges in filament voltage and makes it possible to measure smallditferentialvoltages at high mean levels. With supply voltages of +450and 150, inputs from minus 100 to +150 volts may be measured. withoutchangein. grid current. Differential voltages of 0.1 volt or less at alevel of 150 volts may be measured with the same accuracy as at groundlevel. The grid current can be less than 10- amperes with on dinarytubes. V

The types and dimensions of the components in Figure In Figure 6 I showa circuit which provides a gain of more than unity whereas the other,circuits. provide for a unity gain. in this case a resistor 53 replacesthe gas tube 44 placed between the cathode of the first cathode followervacuum tube and ground. The cathode of cathode follower 33 is connectedto one side ofv the resistor 53 and the other side of the resistor isconnected to ground. The resistor has intermediate taps, tap 54 nearestto the cathode of the cathode follower vacuum tube 33 being connectedthrough plate load resistor 25 to the anode of input vacuum, tube 21,tap 55 more remote from the cathode of cathode follower of vacuum tube33 being connected to the cathode of amplifier tube 24, and tap 56 stillmore remote from the cathode of cathode follower vacuum tube 33 beingconnected tothe cathode of input vacuum tube 21. Output terminal 27 isconnected to the cathode of cathode follower vacuum tube 33 and theinstrument 28 is connected between output terminal 27 and outputterminal 30 which is connected at an intermediate tap on bucking voltagesource 35.

As cathode follower vacuum tube 33 draws current it develops a voltageacross resistor 53 which suppliesthe bias for the input tube, theamplifier and the cathode follower vacuum tube and. the B voltage forthe input vacuum tube via the connection to the anode of the inputvacuum tube through plate load resistor 25. The output terminal 27 isconnected at a point on resistor 53, which is near the connection tothecathodc, of cathode follower vacuum tube 33 or preferably actually tothe cathode as shown, thus giving greaterthanunity gain,

It will be. evident. that the grid current of; the input vacuum tube canbe made Zero by: setting the-bird of the input vacuum tube to a level atwhich the positive ion current (the negative grid current) becomes asgreat as the positive grid; current, making the net current zero. Thisis about 1 to l.8 volts for oxide coated unipotential cathodes attemperaturesof L000, to 1},1, )0 K. This is the floating grid potentialwhich the grid assumes if disconnected. 1

By a combination of positive and negative feedbacks the potentialsbetween the grid and cathode, and between the anode and. cathode and.the current drawn: from anode to cathode and grid to cathode do notchange appreciably with a large change of the input voltage to the inputtube. The input signal maybe many times as large as the voltage fromanode to. cathode or the supply. voltage for the input tube withoutusing input voltage dividers.

In view of my invention and disclosure variations and modifications to.meet individual whim or particular need will doubtless become evident toothers skilled in the art, to obtain all or part of the benefits, of myinvention without copying the structure shown, and I therefore claim allsuch insofar as they fall within thev reasonable spirit and scope of myclaims.

Having thus described my invention what I claim as new and desire tosecure by Letters- Patent" is:

1 In an amplifier drawing minimal input current for a large rangeofinput voltage, input vacuum tube means having an anode, cathode andcontrol grid, amplifier vacuum tube means,having an anode, cathode andcontrol grid; cathode follower vacuum tube means having an anode,cathode and control grid, an input vacuum tithe. load resistor, anamplifier vacuum tube load resistor, input vacuum tube biasing means,amplifier vacuum tube biasing means, cathode follower vacuum tubebiasing means, a source of Bvoltage, means for supplying voltage to theanode of theinput vacuum tube means, an output load device, meansconnecting the control grid-ofthe input vacuum tube means to one sideof-the input, means connecting the anode of the input vacuum tubemeansto one side of the input vacuum tube loadresistor, means connectingthe other side-oftheinputvacuum tube load resistor through the meansforsupplying voltage to the anode of the input vacuum tube to a pointresponsive to the variable potential of the cathode ofthe input vacuumtube means which follows the potential of the inputsignal, such pointbeing free from ground connection, means connecting the anode of theinput vacuum tube means tothe control grid of the amplifier vacuum tubemeans, means connecting the cathode of the amplifiervacuum tube meansthrough the amplifienvacuurn tube biasing means ing the anode of theamplifiervacuum tube means to the 7 control grid of the cathodefollowervacuum tube-means, means connecting the anode of the amplifiervacuum tube means through the amplifier vacuum tube load resistor to thepositive side of'the B source, means connecting the negative side of theB source through the output load device to the point responsive to thevariable potential of the cathode of the input vacuum-tube means, meansconnecting the cathode of the cathode follower vacuum tube through thecathode follower tube biasing means to the point responsive to thevariable potential of" the cathode of the input vacuum tube, meansconnecting the anode of the cathode follower vacuum tube means to thepositive side of the B source and means con.- nectingthe side of theoutput load device which isremote from the cathode through the sourceofbias for the input vacuum tube means to the other side of the. input.

2. An amplifier device comprising a pair of amplifiers each conformingwith claim I. l

3. An amplifier according toclaim 1, in which the-out.- put load devicecomprises an electrical measuring in? strument.

4. An amplifier according to claim 1, wherein. said cathodefollower-vacuum tube biasing means. comprises a gas filled tubeinterposed between the cathodeof the cathode follower vacuum tube meansand the point: responsive to the variable potential of the cathode ofthe input vacuum tube.

5. An amplifier circuit comprising an input signal amplifying meanshaving an input element for receiving an input signal, an outputelementand'a referencev element, an intermediate signal amplifying means havingan input element, an output element and a reference element, said inputelement of said intermediate signal amplifying means being responsive tosaid output element of said input signal amplifying means, alt-outputsignal amplifying means responsive to said output element of saidintermediate ,signal amplifying means, an output circuit means coupledto saidoutputsignal'amplifying means, first feedback means responsive tosaid output circuit means when an input signal is present a; said inputelement of said input signal amplifying "1. .4115, for feedin a signalfrom id pu circ t eans. nrhase wit s d np s a to said output element, ofsaid, input signal amplifying means, second feedback means responsive tosaid output circuit means when said input signal is present at saidinput element of said input signal amplifying means for feeding a signalfrom said output circuit means to said reference element, and a thirdfeedback means responsive to said output circuit means when an inputsignal is present at said input element of said input signal amplifyingmeans for feeding a signal from said output circuit means to saidintermediate signal amplifying means which reinforces the signalreceived by the input element of said intermediate signal amplifyingmeans from the output element of said input signal amplifying means.

6. The amplifier circuit of claim wherein in the presence of an inputsignal of a given amplitude and polarity at said input element of saidinput signal amplifying means said first, second, and third feedbackmeans feed signals of said given amplitude and polarity to said outputelement and said reference element of said input signal amplifying meansand to said intermediate signal amplifying means respectively.

7. An amplifier circuit comprising an input signal amplifying meanshaving an input element for receiving an input signal, an output elementfor transmitting a signal and a reference element to which an inputsignal is referred, an output signal amplifying means having an inputmeans and an output means, means for coupling the output element of saidinput signal amplifying means to the input means of said output signalamplifying means, an output circuit means, means for coupling the outputmeans of said output signal amplifying means to said output circuitmeans, first feedback means coupling said output circuit means to theoutput element of said input signal amplifying means for feeding asignal from said output circuit means back to said output element inphase with an input signal present at said input element, and secondfeedback means coupling said output circuit means to the referenceelement of said input signal amplifying means for feeding a signal fromsaid output circuit means 10 back to said reference element in phasewith said input signal present at said input element.

8. An amplifier circuit comprising an input signal amplifying meanshaving an input element, an output element and a reference element, anoutput signal amplifying means having an input means and an outputmeans, means for coupling the output element of said input signalamplifying means to the input means of said output signal amplifyingmeans, a source of reference potential, output circuit means, firstcoupling means for coupling the output means of said output signalamplifying means to said output circuit means, second coupling means forcoupling the output element of said input signal amplifying means viasaid output circuit means to said source of reference potential forfeeding a signal from said output circuit means back to said outputelement of said input signal amplifying means in phase with an inputsignal present at said input element of said input signal amplifyingmeans, and third coupling means for coupling said reference element ofsaid input signal amplifying means via said output circuit means to saidsource of reference potential for feeding a signal from said outputcircuit means back to said reference element of said input signalamplifying means in phase with an input signal present at such inputelement of said input signal amplifying means.

References Cited in the file of this patent or the origlnal patentUNITED STATES PATENTS 2,181,865 Black Dec. 5, 1939 2,363,057 GaylordNov. 21, 1944 2,434,297 Test et a1. Jan. 13, 1948 2,435,579 Francis Feb.10, 1948 FOREIGN PATENTS 707,488 Germany June 24, 1941

